This invention relates to a system for monitoring the performance of HC sensors arranged in the exhaust passage of an internal combustion engine for detecting concentration of hydrocarbons (HC) in exhaust gases emitted from the engine.
Conventionally, a system has been proposed by Japanese Provisional Patent Publication (Kokai) No. 50-47228, which uses an HC sensor arranged in an exhaust passage of an internal combustion engine, to control in response to output from the HC sensor, an amount of fuel and an amount of air supplied to the engine such that the concentration of noxious components (HC) in exhaust gases decreases to the minimum value.
Further, a system for detecting deterioration of a three-way catalyst of an internal combustion engine has been proposed by the present assignee e.g. by U.S. Ser. No. 07 717,247 filed Jun. 18, 1991. The proposed system uses two HC sensors arranged in an exhaust passage of an internal combustion engine respectively at locations upstream and downstream of a three-way catalyst arranged in the exhaust passage, to determine whether the three-way catalyst is deteriorated or not, by comparing outputs from the HC sensors.
However, in general, the performance of HC sensors such as an output characteristic thereof deteriorates due to aging etc. If various controls are carried out based on the output from an HC sensor which is thus degraded in performance, such controls cannot attain required control accuracy.
More specifically, in the above described two systems, if the output from the HC sensor which does not accurately reflect the concentration of HC in exhaust gases is used, accurate air-fuel ratio control cannot be effected, or accurate detection of deterioration of the three-way catalyst cannot be effected.